A fuel cell in which fuel supply spaces each disposed on one side of an electrolyte layer and supplied with a gaseous fuel are communicated in series with each other has been put into practical use. Further, a flow-type fuel cell in which a plurality of fuel supply spaces are formed in a cascade system wherein the fuel supply spaces are connected in series such that the number of fuel supply spaces communicated in parallel with each other is gradually decreased toward a downstream side has also been put into practical use. By the cascade system, the reduction in flow rate on a downstream side of a gaseous fuel due to consumption of the gaseous fuel through the electrolyte layer can be compensated, and stable supply flow rate of the gaseous fuel in the fuel supply spaces can be secured from the most upstream side to the most downstream side.
An air breathing fuel cell in which a polymer electrolyte membrane is used as an electrolyte layer and one side of the polymer electrolyte membrane is in communication with the atmosphere, and which generates electric power by an electrochemical reaction of a gaseous fuel with oxygen from the atmosphere has been put into practical use. Since the polymer electrolyte membrane is not a completely airtight membrane, when a fuel supply space and a space communicating with the atmosphere are formed with the polymer electrolyte membrane therebetween, atmospheric nitrogen will intrude into the fuel supply space by concentration diffusion from the space communicating with the atmosphere. Since the atmospheric nitrogen that has intruded into the fuel supply space lowers the partial pressure of the gaseous fuel in the fuel supply space to lower the power generation efficiency, it is desirable to purge the impurity gas containing nitrogen in the fuel supply space to the atmosphere periodically.
U.S. Pat. No. 6,960,401 discloses a dead-ended fuel cell in which fuel supply spaces connected in a cascade system are purged. The connection of the fuel supply spaces in the cascade system concentrates and stores impurity gas in the fuel supply space located on the most downstream side.
The constant flow of a gaseous fuel formed in all the fuel supply spaces from the most upstream to the most downstream suppresses the storage of impurity gas in a space except the most downstream fuel supply space. Further, when the output voltage of the fuel cell unit generating electric power by using a fuel in the most downstream fuel supply space falls below a predetermined threshold due to the influence of stored impurity gas, a valve disposed on the downstream side in the most downstream fuel supply space is opened. At the same time, the gaseous fuel is injected into the most downstream fuel supply space from the upstream to discharge the concentrated impurity gas from the fuel supply space into the atmosphere.
In the fuel cell described in U.S. Pat. No. 6,960,401, the flow of gaseous fuel from upstream to downstream in the fuel supply spaces connected to each other in series stores impurity gas in the most downstream fuel supply space. The flow of the gaseous fuel into the most downstream fuel supply space precludes the concentrated impurity gas in the most downstream fuel supply space from diffusing upstream.
Therefore, the output of the fuel cell is stopped to discontinue the flow of the gaseous fuel, the concentrated impurity gas in the most downstream fuel supply space will diffuse upstream and fill the whole fuel cell with the impurity gas. Even when the output does not stop, if the output of the fuel cell is small, the impurity gas can not sufficiently be concentrated in the most downstream fuel supply space.
Further, when performing a purge operation in a state in which the impurity gas is not sufficiently concentrated, a large amount of the gaseous fuel is wastefully discharged into the atmosphere. For this reason, a device on which the fuel cell is mounted needs to be designed on the assumption that a large amount of gaseous fuel is discharged.
Moreover, in a state in which the impurity gas is not sufficiently concentrated, it takes a considerable time until the output voltage of the most downstream fuel cell unit falls below a predetermined threshold, which may seriously delay a purge operation. Even when the impurity gas in the most downstream fuel supply space is low in concentration, a critical amount of the impurity gas is stored all over the fuel cell. Increasing output current in this state accelerates the flow of gaseous fuel from upstream toward downstream to discharge impurity gas downstream, thereby rapidly increasing the concentration of impurity gas in the most downstream fuel supply space. This may stop power generation in the most downstream fuel cell unit. Furthermore, in the case of a fuel cell in which all the fuel cell units are connected to each other in series from upstream to downstream, the output of the whole fuel cell may stop.
That is to say, the fuel cell disclosed in U.S. Pat. No. 6,960,401 can perform an effective purge operation in an appropriate timing in applications where the variation of load is small and a large amount of output current needs to be continuously flowed steadily. However, the purge operation is apt to be inappropriate in applications where the start and stop of output are repeated, output current smaller than that of a designed value needs to be continuously flowed, or the variation of load is large. In addition, gaseous fuel will be wastefully consumed by the purge operation.